Hi NW!
First let me say that I think you are doing a good job helping raise your step-daughter! It is clear how much you love her! She will thank you some day.
Second, wow you sure hit a hot button with this one!
Third, you know what I thought when I read your post? What a compliment to you and your husband! She is bringing her mommy to the safe place! She is trying to rescue her mommy and have her be a part of the safe family. She feels safe enough to show her feelings. She is exercising her choice!!
You made a safe enough place for her to have feelings.
Totally agree with you that it may not be the ideal person, and as an adult I might be a little offended too. But go to her mind. This makes sense to her. This is a safe place for her to feel her feelings with no interference from her mom, half-sis, or mom's BF. She is probably creating a "safe fantasy" relationship with her mom. I think it is healthy and safe for her for awhile.
I would let it go so that she thinks she has control. Then in a month I would sit down with her, complement her on her showing of love for family members by putting up pictures and then add that "we really love you and wondered if we could put our pictures up in your room too...". Even if you took the pictures down, leave the one up and then do this with her. I know it seems weird but it is a complement to you and your hubby.
Continue doing the good job loving her!!